1) Field of the Invention
This invention relates to liquid crystalline compounds, and more specifically to liquid crystalline compounds useful .as liquid crystalline display materials. The present invention is also concerned with liquid crystal compositions containing the liquid crystalline compounds and also with liquid crystal displays using the compositions.
2) Description of the Related Art
Liquid crystal displays making use of the electrooptical effects of a liquid crystal have found widespread utility in various equipment led by watches, clocks and handheld calculators and including word processors and television sets. In particular, TN (twisted nematic) liquid crystal displays using the optical anisotropy and dielectric anisotropy of a liquid crystalline substance are employed predominantly.
Characteristics which are now considered to be needed for a liquid crystal usable in a practical liquid crystal display are as follows:
(1) The liquid crystal has a broad liquid crystalline temperature range.
(2) The liquid crystal has low viscosity. It is evident that the response characteristics of a liquid crystal display have a close relationship with the viscosity of its liquid crystalline material. Described specifically, use of a low-viscosity liquid crystalline material is essential for the provision of a high response speed.
(3) Its optical anisotropy (.DELTA.n) conforms with the optical characteristics of the liquid crystal display. The quality of displayed images, such as the visual angle characteristic and contrast of images displayed by the liquid crystal display, are significantly governed by .DELTA.n, so that .DELTA.n must be suitably controlled in order to obtain high contrast and high visual angle.
(4) Its dielectric anisotropy (.DELTA..epsilon.) conforms with the panel drive method. In the TN type, for example, .DELTA..epsilon. must be large to permit a low drive voltage.
(5) The liquid crystal is stable chemically and optically.
No single liquid crystalline compound has been found yet, which can meet all the above requirements in characteristics. It is, therefore, the current circumstances that liquid crystal compositions formed in combination of several liquid crystalline compounds and/or non-liquid crystalline compounds having various characteristics are furnished for actual use.
Conventionally, liquid crystalline compounds have been classified depending on the compounds present in their core portions. Many compounds have been developed and actually employed including, for example, compounds containing benzene as a core, compounds with cyclohexane as a core, compounds having cyclohexene as a core, compounds having pyrimidine as a core, compounds with dioxane as a core, and compounds containing two or more of these moieties as a core. Even if they are used in combinations, it is still impossible to fully satisfy the above characteristics. There is accordingly a long standing demand for the provision of liquid crystalline compounds of a newer type.